Custom fonts CookBook example not working as expected flutter/website#3591 Based on what @jason-simmons mentioned, I created the following python script to "fix" your custom font files to use the correct weights that flutter respects. Create a src directory and put all your fonts in there. Create an empty out directory and the script will place …

ffont.py

import os
from fontTools import ttLib

# Declare your font file names and the correct weights here
to_change = {
    'Galano Grotesque Thin.otf': 100,
    'Galano Grotesque Thin Italic.otf': 100,
    'Galano Grotesque Extra Light.otf': 200,
    'Galano Grotesque Extra Light Italic.otf': 200,
    'Galano Grotesque Light.otf': 300,
    'Galano Grotesque Light Italic.otf': 300,
    'Galano Grotesque Regular.otf': 400,
    'Galano Grotesque Regular Italic.otf': 400,
    'Galano Grotesque Medium.otf': 500,
    'Galano Grotesque Medium Italic.otf': 500,
    'Galano Grotesque Semi Bold.otf': 600,
    'Galano Grotesque Semi Bold Italic.otf': 600,
    'Galano Grotesque Bold.otf': 700,
    'Galano Grotesque Bold Italic.otf': 700,
    'Galano Grotesque Extra Bold.otf': 800,
    'Galano Grotesque Extra Bold Italic.otf': 800,
    'Galano Grotesque Black.otf': 900,
    'Galano Grotesque Black Italic.otf': 900,
}

for file, weight in to_change.items():
    with ttLib.TTFont(os.path.join('src', file)) as tt:
        tt['OS/2'].usWeightClass = weight
        tt.save(os.path.join('out',file))